Acoustic flow meters



Nov. 7, 1961 F. R. HILL ACOUSTIC FLOW METERS Filed May 16, 1957 United States atent 3,607,359 ACUSTC FLW r/ETERS Frank Robert Hiii, South Lincoln, Mass.,rassignor to Raytheon Sompany, a corporation oi' Delaware FiiedMay lo, 957Ser. N 659,711 7 iar'ns. (Ci. 7Bn-ld) This invention relates to the measurement `ot motion and particularly to the measurement of the rate of the relative motion between a fluid and a solid body. This invention is particularly adaptable to a system utilizing two requenciesfor the determination ofrvelocitysuch as that described in my co-pending application entitled Measurement of Relative Velocity Between Fluid and Solid Bodies, Serial Number 477,572, tiled December y 24, 1954,V now abandoned.

in this invention, the acoustic ilow meter operates on the ,rinciple that the velocity of acoustic transmission etween two ixed points in Aatmoving medium `is .equal to the algebraic sum of V,velocity of propagation in the medium and the components of velocity offiow of the medium in a line joining the two fixed points. The difference between the .upstream and downstream velocity or" acoustic transmission as described is therefore twice the velocity of flow ofthe medium in aline joining the twopoints. TVs invention determines the upstream and downstream velocity of acoustic transmissionin terms of two frequencies in a manner described in my'previously referred-to co-pending application and of a greatly improved accuracy. in this invention, opposing transducers Yare not directly across the stream from each other, but

lrather an angle dcnedby a line throughthe transducers and in the direction of i'iow of the medium. The frequencies f1 (downstream) and f2 (upstream) arel arbitrarily assigned and are automatically adjusted togive the same number of wavelengths, '11.1, between transducers l` and 2. The'eiqnessionforV the frequencies flxand f2 are as follows:

m=number of wavelengths in distance d.

c=propagation velocity yoisokund.

7t=wavelength of sound.

v--veiecity of low.

':aiigles between a line through the transducers and l rthedirection of flow.

By subtracting Equation 2 from Equation 1 we obtain:

to f1 -fi--T and solving for v. in EquationZ we obtain:

in the Vrelationship shown above, d, m, and cos 6 are constants with the variable (f1-,2) beingtdirectly proportional to the -ilow of veiocity. The requirements for A,proper measurement oi flow lor" p velocity require the .de-

lCC

termining of the values of the constants and the measuring and recording of the variables. The distance between transducers c! may be measured very accurately by using standard surveying techniques. Thewa'ngle 0 between sound transmission and the direction of ow is determined at the place and time of installation of the transducers and will be constant for a given installation. The remaining constant forl evaluation ism, thevnumber of acoustic wavelengths in the transmission path. Since the water temperatures in a river may vary, for example from near freezing to themiddle seventies, and further, since suspended and, dissolved materials maybe present in the water in varying amounts, the sound propagation velocity may be as low as 4600 feet per second and as high as 5G00 feet per second which means the average velocity is about4800 feet per secendiZOO feet per second. This wide variation of the propagation constant places a severe restriction on the maximum value of m, which will be explained below. For purposes, of example, a properly designed precision phase detector is capable of reliable operation overa range of approximately i degrees, with an accuracy near zero phase of about il degree. i

The phase shift between opposing transducers V,is equal to:

(5 =360 fm where qb is the phase shift in degrees between transducers. Since d=lmt and 7) ff-*Wn* .order to consider the extreme conditionsy under .which theequipmentvwill be operated, which conditions can. be

i shown Vto. exist if the equipment Ais turned oic vin this condition, and now considering that the, equipment is placed in the extreme opposite condition ,whereim-the propagaltion frequencyis c-,Ac,it can beshown thatthe initial phase will be:

Itis now possible to solveforthe allowable phase shift o since by denition it is known kthat Agb is limited to 1*:15()

degreesand @which is .the average velocity, will be'4800 feet persecond andhc, asexplained above isl'liinited Yto i200 feet per second. Solving for qt in Equation 1G 70.

shows that ,the phase shift angle 1 must be limited to 1800 degrees inorder to obtain the stated accuracy. In

of the medium being measured caused` by an error in phase shift g5, it is necessary to differentiate Equationrl with respect to phase shift o which becomes:

Y By assuming the cos 6 is equal to 0.707 and c is equal to 4800, and further by'requiring an accuracy from the system in the order of i001 feet per second and having assumed the accuracy of the phase detector to be il degree, it is now possible to solve for the minimum possible phase shift allowed in this system by substituting in AEquation l2:

In this invention, the problem of providing the small numberl of degrees (1800) needed for accurate determination of the value of m and the large number of degrees Y (680,000) needed to provide accurate determination of the two frequencies over the wide operating range of civ has been solved by the transmission of a plurality of frequencies from one transducer to the other wherein each of the plurality of frequencies is an exact multiple of the main transmitted carrier frequency and where their zero crossing must coincide with the zero crossing of the transmitted carrier frequency. The main or carrier frequency, as in all acoustic systems, must be high enough to give the transducer good Vertical directivity with a reasonable length, whereas the plurality of multiple frequencies or modulator frequencies should be an exact multiple of the main frequency, since they are used to obtain a lower value for m in order to extend the range of operation without introducing ambiguities. YSingle side band transmission may lbe used to limit the response band of the transducer and to allow more power to be transmitted to theY side band. In the embodiment to be described, the modulator frequency is made a constant submultiple of the main or Y quency for each of Ythe frequencies involved, the signal carrier frequency. In a system utilizing this invention, it

is possible either by the processV of division of the carrier frequency to produce the modulator frequencies or by multiplication of the modulator frequencies produce the main or carrier frequency. Y

Further objects and advantages of this invention will be made more apparent as the description progresses. Reference now being made to the accompanymg drawing where -there is shown in block diagram form a system for measuring uid ow suitable for installation on a river, and where a main or carrier frequency and modulation frequencies are transmittedfrom a first transducer to a second-transducer yand where la second main or carrier frequency and suitable modulation frequencies are transmitted from said second transducer tosaid first transducer on a time-sharing basis. The block diagram to be described is an embodiment of the invention that will solve the problem described earlier in the specification. VFor this specific problem, the choice of carrier frequencies is determined by the needed phase shift of 680,000 and the allowed phase shift of only 1,800". The ratio is approximately 378:1, and, since the phase detectors have a stated accuracy il" and a range yof i150", and hence a ratio of 150:1, it can be seen -that a third frequency must be provided which is intermediate both the and low frequencies. The carrier frequency needed is determined by the average velocity of sound and the allowable phase shift. In the accompanying figure, oscillator 10 generates ak Y carrier frequency of 35.2 kilocycles, the output of which is fed to both a modulator 11 and also to a 16:1, divider circuit 12. that produces a 2,200'cycle`per second signal whichin turn, feeds a 32:1 divider circuit 13 which prov duces'a 68.7 cycle perv second signal. The output of di-` vider 113, which produces the 618.7 cycle persecondsignah *i also feeds modulator 11 as does the output of divider 12 that produces the 2,200 cycle per second signal. InY operation, the carrier signal for the main frequency of 35.2

kilocycles is produced in oscillator 10, whereas the intermediate signal from divider 12.V and thelow frequency signal from divider y13 are applied as amplitude modulation signals upon the carrier signal in modulator 11. The signal-from modulatorv 11 is fed through a power amplifier 14 whereit is amplified for transmission before being fed to transducer 15. The acoustic sign-al is beamed directly through the medium being measured to a suitably-placed transducer 16 where the signal is received and fed to receiver 17 where the carrier signal and'modulating signals are amplified. Since it may be possible under certain conditions for the acoustic signal to be completely blocked, which condition may be caused by oating obstacles or even possibly deep wakes generatednby tug boats, receiver Y .i 17 provides a minimum level gate whichis connected ,to a Y servo amplifier 18 Yin such a manner that said servo amplifier 18 is disabled during the time when receiver 17 receives no usable 'sig-nal' from the receiver transducer 16.r`

detector 20-1and high-'frequency detector 21 where the individu-a1 modulating frequencies are detected and amplified. In order to determine the phase difference Vbetween `the transmitter frequency and the received frefrom low-frequency detector 19 is fed Ito a low-frequency phase detector 22 and in a similar manner the signal from intermediate-frequency detector 20 is fed toV an intermediate-frequency phase Ydetector 23 and the signal from highfrequency detector -21 to -a high-frequency phase detector 24. In order to provide a reference from which to determine the phase shiftfor these three received signals, a portion ofthe'transmittedv signal isV fed `from power amplifier 14 to an attenuator circuit 25 which, in turn, feeds the signal to alow-.frequency detector and amplifier 26 where .the low-frequency signal being transmitted is detected andY amplified and fed tolow-frequency phaseY detector 22'inrorder'to allow ,comparisonV of the phase diiference ybetween the transmitted signal and the received signal to be made. In a similar manner, attenuator 25 also feeds an intermediate-frequency detector 27 K which is oo'n-V nected to and serves as a basis for Iintermediate-frequency phase detector 23 to compare the phase difference for the intermediate-frequency phase signal that is transmitted and with that received. Similarly, attenuator 25 is connected to high-,frequency detector 28 which is connected to high-V frequency phase detector 24 and functions as detectors 2.2 and 23. v

The outputs of phase detectors 22 and 23 are fed to a i first priority circuit 29, that, in normal operation, will select the output of low-frequency phasedetector 22'an`d will feed said signal directly to servo amplifier 18 and at the same time continually test the output of Iboth the lowfrequency detector 122 and intermediate-frequency phase detector Z3 in s uch a vmanner that'when the output Vof the low-frequency phase detector falls belowY a level equivalent to approximately 4, the first priority circuit 29 will select the output of the intermediate-frequency,phase .detector 23 and feeds this signalf to a second priority rcircuit 30, 'while at the same time disabling the circuitV directly fed i to servo -ampliner 1S. High-frequency detector 24 is Yalso fed to second priority circuit 30, which circuit-isnormally disabled unless there isV an output signal from rst priority circuit 29. Should a signal appear at the input of second performs the same i ciated equipment.

priority circuitvil from fufst priority circuit 29, said signal `would-beindicative of an output from intermediate frequency-phase @detector 23 and, under normal conditions, this signal willbe fed directly to servo amplilier 1S while at the same time second priority circuit 3S will continually servo amplifier from the high frequency phase detector 24. back te the intermediate frequency phase detector 23 until the. frequency has been sufhciently adiusted so that no .ambiguity can enter in the operation of the high fre- .quency phase detector 24.

ervoamplilier i8 controls a motor drive unit Si which 1s mechanically linlied through a gear train 32 to oscillator '16, ther-shyv providing accurate control of the carrier frequency .generatedin oscillator lil, which frequency is f1 as described Equation l. The signal gene-rated in oscillater tis also fed to a miser 33.

The description up to this point has been concerned with only the irst mode of operation which includes the transmitting .of .a signal fromtransducer l5 that is re- .ceived by .transduceril As explained previously, this will produce only the f1 frequency `and in order to produce f2 frequency, which. has been arbitrarily defined as `the upstream frequency in Equation 2, itwould be vnecessaryito vprovide a complete duplicate of the apparatus described herein. It has `been discovered that proper ope-ration will occur if suitable gating circuits are used which time-share the equipment in such a .manner that in the rst instance transducer transmits and transducer 16 receives, yand in the, second instance, transducer 16 transmits md transducer .15..receiyes With the equipment used in this fashioinpit is only. necessary to duplicate the carrier oscillator 4and associated motor control units andprovidethe .necsary gating circuits necessary to cause the switching actions to take placewhich .represent averydiiinite savings in dollar value of equipment necessary to practice this. invention.

ln theaccompanying drawing, the referred-to gating circuits that allow the time-sharing of vthe equipment are illustrated as vsingle-pole, double-throw switches, such as switch V1A located at vthe output .of oscillator l0, .which switch, when operated, disables the outputY of oscillator lll and inserts the output ofoscillator 34, which is similar to that-of oscillator 10, intodivider circuit 12 and asso- Single-pole, double throw switch B is vlocated at the output of pcweramplier 14, and when operated has fthe eect of `transferring the transmitted Vpower from 'power-amplifier l@ to transducer 16,thereby making said transducer'l the transmitting transducer.

`Single-pole,double-throw switch C, which is located at the input Vto receiver 17, ,when operated, has the effect of disconnecting transducer 16 from receiver 17 and connecting-said transducer 15 to said receiver, therebyniaking transducer l5 the receiving transducer. Switch D is a single-pole, double-throw switch in the output of servo amplifier i8 and when operated disconnectsvthe output of saidservo amplifierV from controlling vmotor drive unit 3l andinstead feeds -the-outputof said servo amplier 18 to drive lrnoitorfCtS which mechanicallycontrols oscillator 3d through gear train 36. lnthis manner,

with switches A., B, C, and D operated, transducer-16 will be transmitting and transducer l5 will be the receiving transducer, and .which will cooperate with the associatedV circuit in the same manner as heretofore described,

all for the specific purpose of determining the frequency be ,measured with-respect. to the other two systems.

in oscillator .34, which frequency will be f2, as described .in .Equation 2. The output of oscillator 34 is `also fed to mixer 33 vwhere the diiferencefrequency between f1 .fromcscillator i@ and f2 from oscillator 34.is derived. VThis difference frequency lis measured .by a frequency meter Swhich is connected to the output of mixer 33, which difference frequency is `autoiriatically recorded ou a suitable recorder 38.

This completes the description of the embodiments of `this invention described herein. However, many modications and advantages. will be apparent to persons skilled .in the art without departing from the spirit and scope vof this invention.

For eXample,.in certain installations it might he more `advantageous to .use two complete duplicated systems each. having a separate transmitting transducer and a separate receiving transducer rather than use the gating switches illustrated 4and .described in this embodiment. Further, it is entirely .conceivable thatfour separate systems may be used .as described and illustrated herein with the only .difference being that two of the systems would be placed `at right, angles inthe medium to In .thismanner itwould bepossihlein harbors .and rivers 4-ton'ieasure .both magnitude and direction of iiow. Ac-

cordingly, it is desiredthat t invention not be limited to the particular ,details of the embodiment disclosed herein except as delined by theappended claims.

- What is claimed is:

1. Means for measuring the ratte of relative motion, as between iluid and solid bodies, including a first, transmittingrand receiving means. positioned at a giirst predeterminedlocation onv vs aidsolid body, a second transmitting and receivingy means lpt )si-tioned at a second predetermined location en said solid body, v.said `irst-, and said second. locations thereby ldefining a direction making a `predetermined angle other than with respect to the directionof ilow of said fluid, means forgenerating a first carrier signal having aiirst predetermined frequency, ,means'for generatingya first plurality of additional signals first carrier frequency, ,means for generating` a, second ycarrier signal-hazing a second predetermined frequency,

each .having afrequency which isa subniultiple of said means for generatinga second.plurality of additional .signals each .having a frequency .whichis a submultiple of. said seccndrarrien frequency, SadirS-t transmitting means transmittingsaid `first lcarrier signal and* said firs-t i plurality vof additional frequencies from said rstdlocation to vsaid receiving means at Said Second Baastian, Said Second transmitting means transmitting said second carrier signal 5o. second location to said lreceiving Ameans atsaid iirst locatio n, said transmission paths entendingthrough said uid,

land,saidsecond.plurality of additionatsignals from said means responsive to said received signals and said transv rniitted signals for adjusting the frequencicsof said car- A between saidiirst andsecond locations, and means for comparing the fr equnecyy ,of said. irst carrier signal with .thefrequencyof saidsecond carrier. signal for providing al signal proportional to said rateof relativernotion.

2. Means formcasuring the rate of relative motion, as between fluid and solid bodies, including a first transmitlocation-.on said, solidbodypsaid4 first and said second carriersignal having a first predetermined frequency,

means for generating arstpluralitycf additional signals each having a frequency which is a submultpile of said irst. carrier frequency, means for generating a second Carlier signal having a second predetermined frequency, means for generating asecond plurality of additional signals each having a frequencywhich vis a submultiple 7 Y of vsaid second carrier frequency, said first transmitting means transmitting said first carrier signal and said first plurality of additional frequencies from said first location to said receiving means at said second location, said second transmitting means transmitting said second carrier signal and said second plurality of additional signals from said second location to said receiw'ng means at said first location, said transmission paths extending through said fluid, means for comparing each of said received signals with its corresponding transmitted signal, means responsive to the output of VVsaid comparing means for adjusting the frequencies of said carrier signals to provide the same number of wavelengths between said first and second locations, and means for comparing the frequency of said first carrier signal with the frequency of said second carrier signal for providing a signal proportional to said rate of relative motion.V l

3. Means for measuring the rate of relative motion, as between fiuid and solid bodies, including 4a'first transmitting and receiving means located at a first predetermined point on said solid body, a second transmitting and receiving means located at a second predetermined point on said solid body, said first and saidsecond points thereby definin-g a line making a predetermined angle other than 90 with respect to the direction of flow of said fluid, means for generating a first carrier signal having a first predetermined frequency, means for generatingy a first plurality of additional signals each having a frequency which is a submultiple of said first carrier Ifrequency, means for generating a second carrier sign-al having a second predetermined frequency, means forv generating a second plurality of additional signals each having -a frequency which is a submultiple of said second carrier frequency, said first transmitting means transmitting said first carrier signal and said first plurality of additional frequencies in a downstream direction from said first point to said receiving means at said second point, said second transmitting means transmitting said second carrier signal and said second plurality of additional signals in an upstream direction from said second point to vsaid receiving means at said first point, said signals being transmitted downstream on a time-sharing basis with said signals being transmitted upstream, said transmission paths extending through said uid, means for comparing each of said received signals with its-corresponding transmitted signal, means responsive to the output of said comparing means for adjusting the frequencies of said carrier signals to provide the saine number of -wavelengths between said first and second points and means for comparing the frequency of said first carrier signal with the frequency of said second carrier signal for providing a signal'proportional to said rate of relative motion. Y

4. Means for measuring the rate of relative motion, as -between uid and solid bodies, including `a first transmitting and receiving means positioned at a first predetermined location on said solid body, a second transmitting and receiving means positioned at a second predetermined location on said solid body, said first and said second locations thereby defining 4a direction making a predetermined angle other than 90 with respect to the direction of ow of said fiuid, means for generating aV first carrier signal having -a first predetermined frequency, means for generatingta first plurality of modulating signals each having a frequency which is a submultiple of said first carrier frequency, means for generating a second carrier signal having a second'predetermined frequency, means for generating a second plurality of modulating signal-s eachV having ya frequency which is a submultiple of said secondV carrier frequency, said first transmitting means transmitting said first carrier signal and said first plurality of modulating frequencies from said first location 4to said receiving means at said second location, said second transmitting means transmitting said second carrier signal and said second plurality of modulating signals from said second location to said Vreceiving ,Y

means-at said first location, said transmission paths extending through said fiuid, means Vfor comparing each of said received signals with its corresponding transmitted 5 signal, meansY responsiverto the output of said comparing ,Y

means'for adjustirigfthe frequencies of said carrier sigo nals-to providethe same number of wavelengths betweenY said firstV and second locations, and means for compar'- ing the frequency of s-aid first carrier'signal withrthe signal proportional to said rateof relative motlon.

5. Means for measuring the rate of relativemotion, as

Y between fiuid and Ysolid bodies, including ra rst transmitting and receiving means positioned at a Vfirst predeter-V mined location Aon said solid body, a second transmitting Y Y t Y.

and receiving means positioned at a second predetermined determined frequency, means for generating from said vsecond carrier signal a second plurality of additional Vsignals each having a frequencywhich is a submultiple of said second carrier frequency,:said rst transmitting means transmitting s-aid first carrier signal and said first plurality of additional frequencies from said first Vlocation to said receiving means at said second location, `said second trans-A mittingmeans transmitting said second carrier signal and said second plurality of Vadditional signals from said second location to said receiving means at said first location,

said transmission paths extending through said uid,

means for comparing each of said received signalstwitlrY its corresponding transmitted signal, means responsive to the output of said comparing means 'for adjusting the frequencies of said carrier signals to-providethe same number of wavelengths'VV between said vfirst and second locations, and means for comparing the frequency of i said first carrier signal with the frequency of said second carrier sign-a1 for providing a signal proportional to said rate lof relative motion. Y t 'Y 6. Means for measuring theirate of relative motion, as between fluid and solid bodies, including a first transmitting and receivingmeans positioned ata frstpredetermined location'on-said solid body, a second transmit- 'Y ting and receiving means positioned at a secondV predetercf'rnined location onsaid solid body, said first and said secondlocations thereby defining a direction making a Y predetermined angle other than 90"v with respect to the direction of flow of said uid, means for generating aV first carrier signal having a first predetermined frequency, means for generating'V a first plurality of additional signals each having a frequency which is a submultiple of said first carrier frequency, means for generating `aV 6 second carrier signal having a secondpredetermined fre- -ditional signals each'havingia Vfrequency'which is a submultiple of said second carirer frequency, said first transmitting means transmitting said vfirst carrier signal and said first plurality of additional frequencies from said first location to said receiving lmeans ,at said second point, said 'second transmitting means transmitting said second carrier signal and said second pluralityof additional sig- Ynals from said second :location to said receiving means at said first location, said transmission paths extending Vthrough said fluid, Vmeans for comparing the phase of each of said received signals with its corresponding transmitted signal, means responsive to the output of said phase comparing means for adjusting the frequencies of said carrier sign-als to provide the same number of wavelengths betweenv said first and second locations, Yand frequency ofsaid second carrier signal for providingt'a" generating a second carrier signal having a second pre-VV quency, means forgenerating a second plurality of ad- Y means for comparing the frequency of said first carrier signal with the frequency of said second carrier signal for providing a signal proportional to said rate of relative motion.

7. Means for measuring the rate of relative motion, as between iiuid and solid bodies, including a first transmitting and receiving means positioned at a first predetermined location on said solid body, a second transmitting and receiving means positioned at a second predetermined location on said solid body, said first and said second locations thereby defining a direction making a predetermined angle other than 90 With respect to the direction of flow of said iiuid, means for generating a first carrier signal having a first predetermined frequency, means for generating a first plurality of additional signals each having a frequency which is a submultiple of said first carrier frequency, means for generating a second carrier signal having a second predetermined frequency, means for generating a second plurality of additional signals each having a frequency which is a subrnultiple of said second carrier frequency, said first transmitting means transmitting said first carrier signal and said first plurality of additional frequencies from said first location to said receiving means at said second location, said second transmitting means transmitting said second carrier signal and said second plurality of additional signals from said second location to said receiving means at said first location said transmission. Ipaths extending through said iiuid, means for comparing the phase of each of said received signals with its corresponding transmitted signal, means responsive to the smallest phase difference of the highest frequency transmitted and received for adjusting the frequencies of said carrier signals to provide the same number of wavelengths 'between first and second locations, and means for comparing the frequency of said first carrier signal with the frequency of said second carrier signal for providing a signal proportional to said rate of relative motion` References Cited in the file of this patent UNITED STATES PATENTS 2,480,646 Grabau Aug. 30, 1949 2,708,366 Blocher et al. May 17, 1955 2,724,269 Kalmus Nov. 22, 1955 2,826,912 Kritz Mar. 18, 1958 2,865,196 Bordenave et al. Dec. 23, 1958 

